340 upgrades.

72- J heads will respond the same way as Xs to good work. The work suggested is all good, I would just add that the intake face of the head may need to be milled to get an intake to fit and seal properly. Overlooking that can cost a lot of power and give you headaches later. I seriously doubt the factory chambers are 72cc. I'd guestimate closer to 74ccs, if they are indeed virgin. Also, your pistons will be way down in the bores. I'd also guestimate the true static ratio is closer to 8:1 then 9:1. Milling will get back some of this, but the only way to know for sure, is to cc your heads after the work is done, and measure the deck height, then figure the ratio out that way. I'd change the valves to 2.02s to move the seats out, but that will lead to more $$ spent if they are indeed 1.88s (which I seriously doubt). The XE268 will run quite well in it, but if you dont plan to touch much, I'd rather use the XE262 in it. I've also run the Comp Magnum 268H in one of my 340s and had excellent results. I ran it with the factory '71 Tquad intake, and a '72 Tquad carb, and 1 5/8 headers. Passed emmisions, ran pump gas for 2 years in an E body stick w/3.23s.